Textastic is already configured to be able to open all text files, but if a file extension is not registered in the system, iOS does not know that it is a text file.
So it looks like the ".config" file extension is not known to the system so that it doesn't know that it is a text file and thus won't open it in Textastic.
So it looks like the ".config" file extension is not known to the system so that it doesn't know that it is a text file and thus won't open it in Textastic.